1. Field of the Invention
The invention relates to the output buffer portion of integrated circuit devices. More particularly, the invention relates to the control of the impedance of an integrated circuit device output buffer.
2. Description of the Related Art
The increase in the operating speeds of integrated circuits (ICs) has resulted in faster rise and fall times of output voltages in response to inputs generating more system switching noise. Controlling output edge rates and output impedances is required to minimize this noise when transmitting data, e.g., at high rates. The characteristics of output voltages typically are a function of, e.g., temperature variations, process variations and voltage variations (i.e., PVT variations). Another important factor includes the actual load presented to an output terminal. For applications such as adding devices to transmission lines, it is desirable for the impedance of the output driver as seen by the PAD to be constant despite the output driver output waveform, i.e., despite varying output driver output voltage values. Furthermore, it is desirable for the impedance of the output driver to match the load impedance of the transmission line. See, e.g., Gabara et al., "Digitally Adjustable Resistors in CMOS for High-Performance Applications", IEEE Journal of Solid-State Circuits, Vol. 27, No. 8, August 1992, pp. 1176-1185.
Conventional systems use, e.g., arrangements that include constant current sources and some type of analog circuitry to address the problems of variations of the output characteristics as a function of load. However, such conventional systems typically require a higher power drain due to the use of the constant current sources and analog circuitry fabricated on the integrated circuit. With most complementary metal-oxide semiconductor (CMOS) systems, such arrangement is relatively difficult to implement and often requires the use of bipolar-CMOS (BiCMOS) technology in order to realize these functions.
Alternatively, conventional integrated circuit (IC) output buffers include, e.g., a pre-driver stage and an output driver stage. The output driver stage includes a plurality of control bit terminals and a PAD operably connected thereto. Conventional output drivers use single transistors as the pull-up and pull-down resistors within the output driver stage. However, the performance of these pull-up and pull-down resistor arrangements typically is non-linear across the operating voltage swing at the PAD. Also, the non-linear performance is dependent on external factors such as the aforementioned PVT variations. See a discussion of such problems, e.g., in U.S. Pat. Nos. 5,194,765 and 5,243,229.
Therefore, a need exists for a method and apparatus for controlling the output impedance such that it can be maintained within acceptable limits independent of the voltage swing at the PAD and independent of PVT variations.